PSI - Issue 54

Magdalena Eškinja et al. / Procedia Structural Integrity 54 (2024) 123–134 M. Eškinja et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 10. Kernel average misorentation (KAM) maps for HMoS in a) single Q&T condition and in b) double Q&T condition. Second possible origin of the improved HE resistance is change in dislocation density. The dislocation density of the martensite structure decreases with higher tempering temperature what can have positive influence since dislocations are known as reversible hydrogen trap sites. Therefore, lower amount of dislocations can be correlated with lower diffusible hydrogen in the material microstructure. In our case for steel with higher Mo content dislocation density was reduced after double Q&T treatment. Another possible trapping site in the tempered martensite can be cementite/matrix interface, however there was no considerable cementite observed in the microstructure of HMoS and cementite as possible cause of improved resistance can be eliminated. Finally, gaseous hydrogen charging under H 2 /H 2 S/CO 2 + 3.5% NaCl conditions (Figure 11) confirmed superior behaviour of double Q&T high Mo steel and did not show any fractures, although the content of hydrogen uptake was slightly higher compared to single Q&T condition.

Fig. 11. Data of gaseous hydrogen charging experiments for HMoS in single and double Q&T condition in H 2 /H 2 S/CO 2 + 3.5% NaCl environment conditions. Specimens were loaded in elastic region (80% YS).